1. Field of the Invention
The present invention relates to a particle beam therapeutic apparatus capable of irradiating a planned dose by repeating the irradiation of a particle beam onto a diseased part.
2. Description of the Related Art
As a process of forming the irradiation field of a particle beam in a conventional particle beam therapeutic apparatus, there have been known a single painting method of performing irradiation over an entire diseased part by moving the position of irradiation of a particle beam each time a desired planned dose at a desired position is reached (see, for example, a first patent document: Japanese patent application laid-open No. 2002-191709) and a repainting method of irradiating a desired planned dose by scanning the position of irradiation of a particle beam in an amount of dose equal to one severalth of a desired planned dose so as to trace a predetermined track or locus with respect to a target area in such manner that the tracks or loci of the particle beam thus irradiated are repeatedly overlapped one another (see, for example, a first non-patent document: Masataka Komori, et al, “Optimization of Spiral-Wobbler System for Heavy-Ion Radiotherapy”, “Japanese Journal of Applied Physics”, 2004, Vol. 43, No. 9A, pp. 6463-6467).
As a track or locus employed in the repainting method, there are a circular track drawn by the point of a rotating vector with a fixed magnitude as in a wobbler method, and a spiral track drawn by the point of a rotating vector whose magnitude is modulated according to a function of the square root of time as in a spiral wobbler method.
In this spiral wobbler method, spiral tracks or loci are drawn so as to raise the uniformity of dose distribution as compared with the wobbler method. In this case, in order to scan the position of irradiation of the particle beam by applying magnetic fields to the particle beam in two axis directions (hereinafter referred to as an X-axis direction and a Y-axis direction) orthogonal to the direction of travel of the particle beam, excitation currents IX, IY obtained from the following expressions (1) through (4) are supplied to two electromagnets to be magnetized in orthogonal directions. The magnitude of the X-axis exciting current IX changes according to the product of a value that varies from 0 to 0 passing through a maximum value in a spiral period TS and a value that varies according to a sine function of a wobbler angular velocity ω. Also, the magnitude of the Y-axis exciting current IY changes according to the product of a value that varies from 0 to 0 passing through a maximum value in the spiral period TS and a value that varies according to a cosine function of the wobbler angular velocity ω.
                              I          X                =                  A          ⁢                                                    2                ⁢                                  (                                      t                    -                                          nT                      S                                                        )                                                            T                S                                              ⁢                      sin            ⁡                          (                                                ω                  ⁢                                                                          ⁢                  t                                +                                  ϕ                  0                                            )                                                          (        1        )                                          where          ⁢                                          ⁢                      nT            S                          ≤        t        <                              (                          n              +              0.5                        )                    ⁢                      T            S                                                                                        I          X                =                  A          ⁢                                                    2                ⁢                                  {                                                                                    (                                                  n                          +                          1                                                )                                            ⁢                                              T                        S                                                              -                    t                                    }                                                            T                S                                              ⁢                      sin            ⁡                          (                                                ω                  ⁢                                                                          ⁢                  t                                +                                  ϕ                  0                                            )                                                          (        2        )                                          where          ⁢                                          ⁢                      (                          n              +              0.5                        )                    ⁢                      T            s                          ≤        t        <                              (                          n              +              1                        )                    ⁢                      T            S                                                                                        I          Y                =                  A          ⁢                                                    2                ⁢                                  (                                      t                    -                                          nT                      S                                                        )                                                            T                S                                              ⁢                      cos            ⁡                          (                                                ω                  ⁢                                                                          ⁢                  t                                +                                  ϕ                  0                                            )                                                          (        3        )                                          where          ⁢                                          ⁢                      nT            s                          ≤        t        <                              (                          n              +              0.5                        )                    ⁢                      T            S                                                                                        I          Y                =                  A          ⁢                                                    2                ⁢                                  {                                                                                    (                                                  n                          +                          1                                                )                                            ⁢                                              T                        S                                                              -                    t                                    }                                                            T                S                                              ⁢                      cos            ⁡                          (                                                ω                  ⁢                                                                          ⁢                  t                                +                                  ϕ                  0                                            )                                                          (        4        )                                          where          ⁢                                          ⁢                      (                          n              +              0.5                        )                    ⁢                      T            s                          ≤        t        <                              (                          n              +              1                        )                    ⁢                      T            S                                                          
However, in the spiral wobbler method, a planned dose is irradiated by overlapping the spiral track or locus drawn in each spiral period TS one over another a desired number or frequency of spiral periods, and the position of each spiral locus thus overlapped is decided by the phase of a corresponding spiral period at its start time point. Accordingly, in order to ensure the uniformity of dose distribution, it is necessary to make the phase of each overlapped spiral locus at the start time point of each spiral period vary in a uniform manner. However, the phase of each spiral locus (hereinafter referred to as a spiral phase) φ at the spiral period start time point is obtained from the following expression (5), so it is uniquely decided by the spiral period TS, the wobbler angular velocity ω, an initial spiral phase φ0, and the number or frequency of spiral periods n.Φ=nωTS+φ0  (5)
Thus, since each spiral phase φ is decided beforehand by the phase φ0, the spiral period TS and the wobbler angular velocity ω at the start time point of a treatment, and hence in order to ensure the desired uniformity, a very large number of spiral loci to be overlapped are employed so that they can be regarded as statistically at random. For example, the above-mentioned first non-patent document, one or more seconds are spent so as to ensure the uniformity of the dose distribution. In this manner, the conventional spiral wobbler method involves a problem that it is necessary to perform irradiation in such a manner that a lot of spiral loci are overlapped each other.
In addition, each spiral phase φ is uniquely decided by the spiral period TS and the wobbler angular velocity ω, so the wobbler angular velocity ω capable of ensuring the uniformity of the dose distribution when the spiral period TS is changed is limited, thus posing a problem that a constraint arises in the selection of the spiral period TS and the wobbler angular velocity ω.
Moreover, since it is necessary to keep one continuous irradiation for one second or more, a particle beam is generally generated from a synchrotron type particle accelerator, which periodically iterates acceleration and deceleration, e.g., a periodic operation is carried out in a period of two seconds, so that the particle beam thus generated is supplied to a particle beam irradiation part at a time between the acceleration and deceleration. Thus, in order to shorten the entire irradiation time, the time to supply the particle beam is decreased as much as possible so as to operate the accelerator in an efficient manner. However, when the time to supply the particle beam is hundreds of milliseconds, the supply of the particle beam is interrupted at a time during one spiral period, thus posing a problem that there is no guarantee for which the uniformity of dose distribution in each operation period of the accelerator is ensured.
Accordingly, there is another problem that when it is intended to prevent the interruption of the supply of the particle beam during the spiral period by increasing the time to supply the particle beam, there arises a constraint in the operation period of the accelerator such as the inability to shorten the supply time of the particle beam.
Further, in a respiration synchronized operation in which the irradiation of the particle beam is turned on and off according to respiration when irradiation is performed on internal organs that are caused to move or deform in accordance with the respiration or breathing of a patient, the supply of the particle beam is interrupted during the spiral period TS, thus causing a problem that there is no guarantee for which the uniformity of the dose distribution is ensured.
Furthermore, when the spiral wobbler method is applied to a layer-stacking conformal irradiation method in which irradiation on a patient is performed by dividing an exposure or irradiation area of the patient into a plurality of subareas or layers in the direction of depth thereof, a planned dose for each of the subareas or layers thus divided becomes smaller, so the number or frequency of spiral periods (hereinafter also referred to as a spiral number or frequency) is decreased, resulting in a problem that a spiral frequency required to ensure the uniformity of dose distribution can not be reached.
Besides, since the number or frequency of spiral periods is necessary to be equal to or more than a predetermined number of times, there is a further problem that when the dose rate is raised, the number or frequency of spiral periods required falls below the predetermined frequency, so the uniformity of dose distribution can not be ensured, and it is necessary to increase the exposure or irradiation time.